Omni-directional antennae are in wide use in communication systems. In such systems, there is an attempt to maximize antenna gain in order to maximize the carrier-to-noise ratio and carrier-to-interference ratio. One method of maximizing omni-directional antenna gain is to stack dipoles in a collinear manner. There are, however, two limitations which must be considered when maximizing antenna gain by stacking dipoles in a collinear manner.
The first limitation is created by physical problems associated with the antenna. It is well known that the power gain of an omni-directional, collinear antenna varies directly with the length of the radiator. Thus, the longer the antenna, the greater the gain. However, simply increasing the length of the antenna to achieve the gain creates other problems. The Federal Communications Commission Rules require aircraft warning lighting on any antenna, itself, that extends more than twenty feet above the top of a tower, building or water tower. Thus, to achieve greater gain, decoupling networks would be required as well as a side ladder for changing the required lights. The additional length, then, creates both electrical and mechanical problems which are not easily solved without greater expenses being required. As the length of the antenna is increased, the physical strength of the antenna must be increased to account for the increased loading under wind loading conditions due to the longer lever arm.
The second limitation on achieving higher gain by stacking dipoles is the compression of the vertical beamwidth as the antenna is increased in length and the gain is increased. The beamwidth decreases by a factor of 2 to achieve each increase in antenna gain of 3 dB. As the beam narrows the criticality of mounting and of maintaining the exact perpendicular position of the antenna, even under wind loading, becomes very important. For a 10 dBd gain antenna, a movement of as slight as 31/2.degree. under high wind conditions, would cause a loss in signal level of 3 dB. It is apparent that such a higher gain antenna must be built more rugged, thus requiring heavier material to be used, in order to form more substantial towers for maintaining the antenna in a fixed position at all times. Further, as the beamwidth narrows, the dead zone or cone of silence beneath the antenna increases. This produces more dead spots. Further, phasing harnesses become more critical as the number of stacked elements increase.
The present invention overcomes disadvantages of the prior art by allowing antenna gain to be increased without increasing the antenna length (height) and without compressing the vertical beamwidth. This is accomplished by limiting the horizontal beamwidth of omni-directional antennae with the use of reflectors. By decreasing the horizontal angle of coverage of the omni-directional antenna, additional gain is achieved without sacrificing vertical beamwidth. Thus, at least four 16 dBd vertically L polarized antennae are formed by taking 10 dBd omni-directional antennae and adding reflectors to limit the horizontal beamwidth to 90.degree.. The vertical beamwidth remains at 7.degree.. Power combining hybrids connect each of the two opposed directional antennae. The output signal from the two hybrids is coupled to a diversity receiver where the amplitude and phase of the signals are adjusted and combined in a constructive manner.
The invention may combine N such directional antennae, where N is an even number .ltoreq.2, with each two opposing antenna outputs being coupled to a passive power combining hybrid. The output of each of the hybrids is then coupled to the diversity receiver.
Thus, the invention provides an improved system having enhanced antenna gain with the use of at least four directional antennae covering a 360.degree. horizontal beamwidth sector and a fixed vertical beamwidth sector and where each individual antenna is arranged to cover no more than a 90.degree. beamwidth sector. A power combiner (hybrid) is coupled to each opposed pair of antennae for combining the signals from the opposed antennae and a single channel diversity receiver is coupled to the hybrid combiners for optimally adjusting the phase and amplitude of each signal and combining the signals.
Each of the directional antennae comprises an omni-directional antenna and a reflector associated with each antenna to limit the horizontal beamwidth to no more than 90.degree..
In the general embodiment of the invention, at least N directional antennae covering 360.degree. horizontal beamwidth sector and having a fixed vertical beamwidth sector are constructed such that each individual antenna is arranged to cover no more than a 360.degree./N beamwidth sector where N=an even number .ltoreq.2. Thus, N=2, 4, 6, 8 and the like.
Further, the equipment arrangement as disclosed herein, has a single channel (frequency) for each receiver. To accommodate multiple channels, a preamplifier/power splitter (multi-coupler) is needed for each signal lead from each combining hybrid feeding each diversity receiver.